1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device and a driving method thereof.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices display an image by controlling the light transmittance of liquid crystal cells in accordance with a video signal. In an active matrix type LCD device such as the one illustrated in FIG. 1, data voltages to be supplied to liquid crystal cells Clc, are switched by thin film transistors (TFTs) formed in respective liquid crystal cells Clc. The TFTs provide active control of the data voltages to achieve an enhancement in the display quality of a moving image.
In FIG. 1, the reference character “Cst” designates a storage capacitor that maintains the data voltage charged in the associated liquid crystal cell Clc, the reference character “D1” designates a data line supplied with the data voltage, and the reference character “G1” designates a gate line supplied with a scan pulse.
A LCD display device having liquid crystal cells as illustrated in FIG. 1 may be driven in accordance with an inversion scheme, in which polarity inversion not only occurs between neighboring liquid crystal cells, but also occurs at intervals of one frame, as shown in FIG. 2, to reduce DC offset components supplied to the liquid crystal cells and to thus reduce degradation of the liquid crystals. When any one of data voltages having opposite polarities is, however, dominantly supplied for a prolonged period of time, image sticking may occur. Such image sticking is called “DC image sticking” because the image sticking occurs as each liquid crystal cell is repeatedly charged with voltages having the same polarity. As an example, such image sticking can occur when data voltages are supplied to the LCD device in accordance with an interlace scheme. The interlace scheme for supplying data includes supplying data voltages to liquid crystal cells on odd horizontal lines in odd frame periods, and supplying data voltages to liquid crystal cells on even horizontal lines in even frame periods. Data supplied in accordance with the interlace scheme is referred to hereinafter as “interlace data”.
FIG. 3 is a waveform diagram that shows an example of interlace data supplied to a liquid crystal cell Clc. In the illustrated example, it is assumed that the liquid crystal cell Clc supplied with the data voltages depicted in FIG. 3 is one of the liquid crystal cells arranged on one odd horizontal line.
Referring to FIG. 3, a positive voltage is supplied to the liquid crystal cell Clc in odd frame periods, and a negative voltage is supplied to the liquid crystal cell Clc in even frame periods. In accordance with the interlace scheme, a data voltage having a high positive polarity level is supplied to liquid crystal cells Clc arranged on odd horizontal lines only in odd frame periods. For this reason, the positive data voltage becomes dominant during 4 frame periods, as compared to the negative voltage, as shown by the waveform in the box of FIG. 3.
FIG. 4 is an image showing the experimental results of DC image sticking occurring due to interlace data. When an original image corresponding to the left image in FIG. 4 is supplied to an LCD panel for a certain period of time in accordance with the interlace scheme, the data voltage, which is varied in polarity at intervals of one frame, exhibits a considerable amplitude difference between the odd frame and the even frame. As a result, when a data voltage having an intermediate grayscale value, for example, a grayscale value of 127, is supplied to all liquid crystal cells Clc of the LCD panel, after the supply of the original image, the pattern of the original image is dimly displayed, as shown by the right image in FIG. 3. That is, DC image sticking occurs.
The moving image display quality of the LCD device may be degraded not only due to DC image sticking, but also due to flicker. Flicker is a periodic brightness difference visible to a viewer's naked eye. Therefore, it is desirable to reduce or eliminate DC image sticking and flicker to enhance the display quality of the LCD device.